The present invention relates generally to improvements in planetarium projector devices and it relates more particularly to an improved phase changing moon projector for planetariums. At present, two types of moon projectors are generally available. One is of the direct casting type which projects the waxing and waning of the moon by directly occulting the projected light, and the other is of the reflecting type which lightens a moon-shaped spherical body with a lamp and projects the waxing and waning of the moon indicated by the reflection of the light from the revolving spherical body. The moon projector of the direct casting type is superior to that of the reflection type because of its simple, compact construction.
The moon projector of the direct casting type employs a condenser lens to collect the diffused rays of light from the lamp, forms an image of the full moon by passing the light through the full moon plate delineating or containing the shape and pattern of the full moon, and directs the image on the screen by a projection lens while effecting the waxing and waning of the moon by occulting the light travelling through the full moon plate with an occulting element located between the plate and the projection lens.
The occulting element contains a concave semi-circular cylindrical surface or U-grooved cylinder which is cut in the rectangular direction, i.e. at the right angle to the axis of the cylinder with a cutter having the same diameter as the cylinder. As the occulting element turns, the projected image changes from the full to the new moon, and then vice versa.
The cycle from a full moon to another full moon can be accomplished by turning the occulting element for 180.degree. . When turning the occulting element further to change the full moon to the new moon, however, the projected image of the moon becomes different from the real one. Therefore, the occulting element is changed with another occulting element having a rectangular inclination against and the same shape as the first element, and the second element turns round to represent the continuous waxing and waning of the moon.
The conventional mechanism for switching the occulting elements uses a spring. When the spring is released from its maximum compression point to replace one element with another, the replacing element strikes against the component which keeps the element fixed at a certain position. The impact tends to jerk the moon projector, and the movement of the element by a compression of the spring tends to distort the image of the moon. When the annual motion is reversed immediately after the change to the element, the normal image of the moon is not projected because of the failure of the occulting element to return. In addition to this disadvantage, when two projectors of the direct casting type, each provided with a respective occulting element, replace each other, their lamps flicker on the occasion of the full moon and the spectator does not fully visualize his presence at the true scene, because the image of the full moon flickers for a moment.
In the case of the conventional projector, the condenser lens has a large aperture because of the need to collect a large amount of light, while its projection lens has a small diameter. Therefore, the light travelling from the condenser lens to the projection lens converges to direct it effectively on the screen.
As explained above and illustrated in FIG. 1 (a), the plan view, and FIG. 1 (b), the frontal cross-sectional view, the conventional occulting element is a semi-circular cylindrical U-grooved cylinder (hereinafter called the "cylindrical occulting element"). Therefore, the waxing and waning of the moon are projected by equalizing the diameter of the cylindrical occulting element with that of the convergent light at the rear of the element in the travelling direction of the convergent light and turning the element around its rotating X-X' .
Accordingly, the diameter of the cylindrical occulting element would become larger as compared with the diameter of the convergent light at the rotating axis X-X' as shown in FIG. 1 (b) and such projected image is unable to reproduce a crescent moon having a sharp cut edge.
Furthermore, if the diameter of the cylindrical occulting element is made equal to the diameter a of the convergent light on the rotary axis X-X' as in FIG. 3, the element does not function well as an occulting element because of the leak of light on the occasion of the crescent moon or the new moon.
Such being the case, the cylindrical occulting element, whatever diameter it may have, makes it possible only to project an image which is different from the real image of the moon.